Wireless technologies give several advantages to industrial automation in terms of gain in productivity and flexibility. Industrial sites are often harsh environments with stringent requirements on the type and quality of cabling. Moreover large sites often require many thousands of cables and it could be difficult to install or engineer additional wires in an already congested site. Thus wireless communication can save costs and time during an installation phase. Furthermore the ad-hoc nature of wireless networks allows for easy setup and re-configuration when the network grows in size.
A requirement of a wireless network is the coexistence of the wireless network with other equipment and competing wireless systems. The WirelessHART standard has been developed to fulfill these demands. WirelessHART is a wireless mesh network communication protocol for process automation applications, including process measurement, control, and asset management applications. It is based on the HART protocol, but it adds wireless capabilities to it enabling users to gain the benefits of wireless technology while maintaining compatibility with existing HART devices, tools and commands. A WirelessHART network may be connected to a plant automation network through a gateway. The plant automation network could be a TCP-based network, a remote I/O system, or a bus such as PROFIBUS. All network devices such as field devices and access points transmit and receive WirelessHART packets and perform the basic functions necessary to support network formation and maintenance. Communications among network devices are arbitrated using TDMA (Time Division Multiple Access) that allows scheduling of the communication link activity.
MAC protocols are the basis of wireless communication technologies, which are used to allocate shared wireless channel among communication nodes. In most cases, MAC protocols can be classified into two types: conflict-free protocols (FDMA, TDMA, CDMA, etc) and conflict-contention protocols (Slotted ALOHA, CSMA, Channel Hopping, etc). Some dominant wireless technologies and their MAC protocols may be briefly summarized as follows:                1 2G cell phone system. The GSM (Global System for Mobile Communications) cellular system combines the use of FDMA (Frequency Division Multiple Access) and TDMA to allow multiple handsets to work in a single cell.        2 2.5G cell phone system. GSM with the GPRS (General packet radio service) packet switched service uses a dynamic TDMA scheme for transferring the data.        3 3G cell phone system. CDMA2000 uses CDMA (Code division multiple access) to send voice, data, and signaling data between mobile phones and cell sites.        4 Bluetooth. Bluetooth[1] packet mode communication uses Channel Hopping for shared channel access among several private area networks.        5 WLAN. IEEE 802.11[2] wireless local area networks uses CSMA/CA for multiple access within the cell.        6 IEEE 802.15.4. IEEE 802.15.4[3] uses TDMA for periodic traffic packets, and CSMA (Carrier Sense Multiple Access) for burst traffic packets.        7 WirelessHART. WirelessHART[4] combines the use of TDMA and Channel Hopping for periodic traffic packets, and Slotted ALOHA for burst traffic packets.        
The above summary review shows that cell phone systems (2G, 2.5G and 3G) just use conflict-free MAC protocols; Bluetooth and WLAN use only conflict-contention MAC protocols. IEEE 802.15.4 and WirelessHART use all of conflict-free and conflict-contention MAC protocols, and different traffic kinds are distinguished in IEEE 802.15.4 and WirelessHART.
There are two kinds of traffic in industrial wireless applications: periodic traffic and burst traffic. Conflict-free MAC (multiple access) protocols, such as TDMA (Time Division Multiple Access), are more suitable for periodic traffic; on the other hand, conflict-contention MAC protocols, such as CSMA (Carrier Sense Multiple Access), are more suitable for burst traffic.
TDMA is a conflict-free MAC protocol, which divides the time axis into superframes. Every superframe is further divided into a lot of time slots and these slots are pre-assigned to different users exclusively. Every user is allowed to transmit freely during the slot assigned to it, that is, during the assigned slot the entire system resources are devoted to that user. TDMA is suitable for periodic traffic which is predictable to pre-assign. However, for the burst traffic packets, TDMA degenerates to function like an ineffective Slotted ALOHA protocol.
The basic idea of Slotted ALOHA is simple: let the wireless user transmit at the beginning of next slot whenever they have packets to be sent. Slotted ALOHA protocol has poor performance because whenever one sender has a packet to transmit it does so without consideration of others. CSMA is an improvement over Slotted ALOHA when it comes to transmitting burst packets. The philosophy of CSMA is that when a sender generates a new packet the channel is sensed and if found idle the packet is transmitted. When a collision takes place each sender reschedules a retransmission of the collided packet to some other time in the future randomly. However, traditional CSMA works in continuous time, which is not directly applied in pure TDMA systems.
IEEE 802.15.4 functions as it were a simple hybrid of TDMA and CSMA. Superframe time is divided into two time phases for TDMA and CSMA respectively, and in different phase, TDMA and CSMA are performed independently. Because CSMA can only be performed in continuous time, this simple hybrid is not used in a pure TDMA environment like WirelessHART. WirelessHART is a pure TDMA system and it distinguished from other, different traffic kinds. However, WirelessHART suffers from poor network performance because of the ineffective Slotted ALOHA-like functioning of its shared transaction slots.
In a patent application US 2009/0129353 entitled “Method for recognizing available channel in IEEE 802.15.4 protocol CSMA/CA mechanism”, assigned to University Sungkyunkwan Foundation [KR], a method of recognizing an available channel in an IEEE 802.15.4 protocol CSMA/CA mechanism is described, in which a Random Backoff Time extension algorithm is used so as to improve performance of slotted CSMA/CA.
In another patent application US 2008/0316966 entitled “Optimizing positions of time slots in a hybrid time division multiple access (TDMA)-carrier sense multiple access (CSMA) medium access control (MAC) for multi-hop ad hoc networks”; assigned to Motorola Inc, a hybrid TDMA-CSMA MAC protocol is described for allocating time slots within a frame having a structure in which transmission time is divided into a first number of actual TDMA time slots and a second number of “virtual” CSMA time slots. Each frame comprises a variable length/duration TDMA portion and a variable length/duration CSMA portion, and the relative percentages of the frame which are allocated for the TDMA portion and the CSMA portion can be dynamically adjusted depending upon the traffic pattern.